The knee is one of the most complex joints in the human body. Anatomically, the knee provides articulation between the femur and the tibia. The medial and lateral femoral condyles are in contact with complementary cartilaginous menisci on the proximal end of the tibia. The relative motions of the femur and tibia about these surfaces of contact are modulated by five ligaments and three muscle groups. The femur and tibia are united by pairs of cruciate and collateral ligaments as well as the patellar ligament. The muscles comprise the hamstring, gastrocnemius and quadriceps groups. Relative motion between the femur and tibia is an intricate synergy of hinge-like flexion and extension, anteroposterior translation (roll-back) and axial rotation.
In normal activities, such as walking and jogging, kneeling, climbing stairs, and getting into or out of chairs, peak loads equal to about five times the weight of the body are applied to the knee joint. Much higher stresses are put on the joint during participation in sports such as tennis, soccer, football, and distance running.
Thus, it is not surprising that knee injuries occur frequently, especially to the menisci and ligamentous supporting structures. Such injuries may lead to chronic swelling and inflammation and eventually to arthritic deterioration. Additionally, arthritic joint destruction may occur as a natural sequelae of cartilage cell senescence or as a result of multiple inflammatory disease processes.
As an arthritic knee may be painful and functionally debilitating, a need exists for prosthetic components to replace the natural joint surfaces which have been damaged by the arthritic process. It has been estimated that approximately 40,000 Americans undergo prosthetic knee replacement procedures per year as a result of arthritic deterioration (Discover, October 1987, pp. 22-23).
The first total knee prosthesis was successfully introduced by Borje Walldius in the early 1950's. The Walldius prosthesis is an example of an "articulated" or "constrained" prosthesis in that the femoral and tibial components are mechanically linked, the muscle/ligament structures of the natural knee not being relied upon to hold the femur and tibia together.
Unconstrained total knee prostheses were introduced in 1971 by Frank Gunston, as represented by the Polycentric prosthesis. The Gunston prosthesis, like many subsequent designs, had femoral and tibial components that were "nonarticulated". In other words, the femoral and tibial components were not connected mechanically, the patient's muscles and ligaments being relied on to hold the knee together in a physiological fashion. More recent unconstrained prosthesis designs feature nearly anatomically shaped components, and are commonly referred to as "anatomical" or "surface replacement" or "cruciate ligament retaining" prostheses. Still other designs impart varying degrees of intrinsic support to the knee joint to compensate for lost ligament support and are termed "semi-constrained" prostheses. Descriptive terms such as "cruciate ligament substituting" or "posteriorly constrained" or "stabilized" are frequently used.
In 1978 it was estimated that more than 80 different prosthetic knees were available (Scientific American, January 1978, pp. 44-51), and there has been intense activity in the areas of prosthesis design since that time. More recent developments in constrained (articulated) total knee prostheses are represented in U.S. Pat. No. 4,358,859 and in U.S. Pat. No. 4,462,120, the latter of which describes femoral and tibial components in which the bearing members may be detached for replacement when worn. U.S. Pat. Nos. 4,309,778; 4,340,978; and 4,470,158 exemplify knee prostheses of the unconstrained type, and U.S. Pat. No. 4,257,129 describes a tibial component featuring a replaceable articulation member.
Currently available knee prostheses typically rely on either poly(methylmethacrylate) bone cement or natural bone ingrowth for fixation. For example, U.S. Pat. No. 4,479,271 describes a tibial component for a knee prosthesis in which a layer of fibrous metal mesh is incorporated to encourage bone ingrowth. Other types of porous-coat surface treatments, designed to encourage natural bone ingrowth, are disclosed in U.S. Pat. Nos. 3,605,123; 3,855,638; and 4,550,448; for example. U.S. Pat. No. 4,551,863 describes fixation using bone cement in combination with bone ingrowth, most specifically as applied to a hip prosthesis.
Failure of fixation of prosthetic components is one of the most frequent mechanical complications associated with total knee arthroplasty (see Proc. Advances in Bioengineering Symposium, December 1978, Published by ASME, New York, N.Y., 1978, pp 49-53 and Clin. Orthop., January-February 1985, pp 34-39, for example). When bone cement fixation is employed, failure often occurs at the bone/cement interface (J. Arthroplasty, vol. 1, 1986, pp 293-296).
Whereas it might be expected that natural bone ingrowth would improve the integrity of the bond between prosthesis and bone and thereby ameliorate the problem, such bony ingrowth is by no means readily achieved, even with the most advanced porous-coated prosthesis surfaces. Apparently, bony ingrowth will not occur readily if there is even slight relative motion between the bone and the adjacent ingrowth surface (see J. Biomed. Mat. Res., vol. 7, 1973, pp 301-311, for example).
Another serious outstanding problem in knee replacement surgery relates to the technical difficulty of removing the prosthetic component. Prosthetic component removal may prove necessary under several circumstances including: structural failure or loosening of one or more components, articulating surface wear, ligamentous instability problems, bony fracture or resorption, or joint infection.
Of particular concern is the removal of the tibial component, in that access to the intramedullary stem, which is commonly employed to improve component fixation and stability, may be prohibitively difficult. Complications associated with tibial component removal include gross bone loss and tibial fracture, as well as extended operating time and creation of particulate metal and plastic debris (J. Arthroplasty, October 1988 Supplement, pp. 587-594).
Various attempts have been made to solve this problem. For example, certain tibial components incorporate slots surrounding the intramedullary fixation stem through which osteotomes may be inserted to break any existing bond to the prosthetic component. Unfortunately, inclusion of slots may compromise the structural integrity of the component and create a predisposition to fatigue failure. Other tibial component designs simply avoid the problem by eliminating intramedullary stems or employing stems of smooth contour so that they may be extracted from bone or a surrounding cement mantel with comparative ease. This option lessens extraction difficulty, but may compromise fixation and long term stability.
It is to the aforecited problems that the present invention is directed. Consequently, it is one object of this invention to provide a tibial prosthesis which facilitates fixation. It is a further objective to provide a tibial component which is readily manipulated to provide clear access to the anchoring mechanism in the event the tibial prosthesis must be revised or removed. It is yet a further objective to provide a modular tibial component which can be employed with a selection of femoral or bearing insert components. Other objectives and advantages of the invention will become apparent hereinafter.